Bromate minimization during ozonation: Mechanistic considerations

Bromate formation during ozonation of bromide-containing natural waters is somewhat inversely connected to the ozone characteristics: an initial fast increase followed by a slower formation rate. During the initial phase mostly OH radical reactions contribute to bromate formation, whereas in the secondary phase both ozone and OH radicals are important. To minimize bromate formation several control options are presented: ammonia addition, pH depression, OH radical scavenging, and scavenging or reduction of hypobromous acid (HOBr) by organic compounds. Only the two first options are applicable in drinking water treatment. By both methods a similar effect of a bromate reduction of approximately 50% can be achieved. However, bromate formation during the initial phase of the ozonation cannot be influenced by either method. Ammonia (NH3) efficiently scavenges HOBr to NH2Br. However, this reaction is reversible which leads to higher required NH3 concentrations than expected. The rate constant k(NH2Br) for the hydrolysis of NH2Br by OH- to NH3 and OBr- was found to be 7.5.10(6) M-1 s(-1). pH depression shifts the HOBr/OBr- equilibrium to HOBr and also affects the ozone chemistry. The effect on ozone chemistry was found to be more important for bromate formation. For a given ozone exposure, the OH radical exposure decreases with decreasing pH. Therefore, for pH depression the overall oxidation capacity for a certain ozone exposure decreases which in turn leads to a smaller bromate formation.